1. Field of the invention
This invention relates to a still-picture TV telephone system, and more particularly to a still-picture TV telephone system in which a still picture signal taken by a TV camera is modulated in phase and amplitude as a signal of voice band and is sent to a telephone circuit during talking over telephone.
2. Description of the Related Art
A TV telephone has been cherished in which a voice and a picture are transmitted simultaneously via a telephone communication circuit. As a matter of fact, however, with the existing telephone communication circuit, a moving picture requiring massive data is difficult to transmit and receive continuously. To this end, a still-picture TV telephone has been developed in which a still picture is transmitted to the other end of the line during talking.
Heretofore, in this type of still-picture TV telephone, it is possible to send a desired still picture or image (hereinafter called "picture"), e.g., a photograph or picture showing the speaker's face and message while the telephone communication is temporarily being stopped during taking in voice ordinarily.
According to this still-picture TV telephone, a picture can be transmitted and received via the telephone circuit, and an advanced telephone communication sending not only voice but visual information.
FIG. 5(a) of the accompanying drawings is a circuit diagram showing this kind of monochrome still-picture TV telephone disclosed in "Telephone Engineer & Management" issued June 15, 1986.
In FIG. 5(a), reference numeral 1 designates a handset; 2, a microphone; 3, a voice input/output circuit; 4, an analog circuit; 5, a matrix switch; 6, a telephone interface circuit; 7, a telephone circuit terminal; 8, a speaker; 9, a keypad; 10, an input/output port; 11, a CPU; 12, a program memory; 13, a modem circuit; 14, a TV camera; 15, an image controller; a picture memory; and 17, a display.
FIG. 5(b) is a block diagram showing the modem circuit 13 of FIG. 5(a). In FIG. 5(b), reference numeral 18 designates a phase/amplitude modulation circuit; 19, a D/A converter; 20, a carrier reproduction circuit; 21, a multiplier; 22, a low-pass filter; and 23, an A/D converter.
FIG. 6 is a sequence diagram showing the picture transmitting procedures of the TV telephone of FIGS. 5(a) and 5(b).
The manner of transmitting the voice and picture signals in the conventional TV telephone will now be described with reference to FIGS. 5(a), 5(b), 6 and 7.
In FIG. 5(a) and 5(b), firstly during talking in voice over the telephone, a voice signal of the speaker is received by the handset 1 or the microphone, and its output is sent from the voice input/output circuit 3 to the analog circuit 4. The output of the analog circuit 4 is transmitted to the telephone circuit terminal 7 via the matrix switch 5 and then the telephone interface circuit 6. Further, the voice signal from a person at the other end of the line is reproduced by the handset 1 or the speaker 8 as it is transmitted to the handset 1 or the speaker 8 along the reverse path.
Meanwhile, in transmitting the still picture signal during talking, before transmission of picture data, as shown in FIG. 6, an ID code is sent to ascertain whether the telephone at the other end of the line is a TV telephone of the same data type, and then the picture data are transmitted after ascertaining the return of an ID code from the telephone at the other end of the line.
Specifically, as the speaker makes a command to transmit a picture by operating the keypad 9 in FIGS. 5(a) and 5(b), the CPU 11 detects this command via the input/output port 10 to supply an ID code to the modem circuit 13 according to a program stored in the program memory 12 beforehand. After having modulated to an analog signal in the modem circuit 13, the ID code is transmitted to the telephone at the other end of the line from the telephone circuit terminal via the matrix switch 5.
Likewise, an ID code from the telephone at the other end of the line is transmitted, along the reverse path, to the CPU 11 which ascertains the ID code.
During that time, the picture taken by the TV camera 14 is quantized in the image controller 15 and is stored temporarily in the picture memory 16, whereupon the quantized data are gradually introduced to and displayed on the display 17 and then are outputted, in response to a command of the CPU 11, to the telephone circuit terminal 7 from the picture memory
In the presence of an ID code or picture data inputted from the telephone at the other end of the line, the matrix switch 5 switches by the carrier signal the direction in which the received signal from the telephone interface circuit 6 is to be transmitted, from the analog circuit 4 when received the voice signal, to the modem circuit 13.
After the received signal has been demodulated in the modem circuit 13, the CPU 11 returns the ID code or stores the data of the picture memory 16, and the image controller 15 gradually introduces the speaker's picture data and those of a person at the other end of the line to the display 17 which then displays the data.
In the absence of any carrier signal from the modem circuit 13 upon termination of transmitting and receiving the picture, the matrix switch 5 again connects the telephone interface circuit 6 and the analog circuit 4 to each other to return to the mode of talking in voice.
In the modem circuit 13 shown in FIG. 5(b), the picture is modulated in the following manner: The digital picture signal transmitted from the CPU 11 is modulated, by the combination of phase and amplitude of sixteen steps shown in FIG. 7, into a signal having a phase and an amplitude which correspond to the brightness of the picture signal.
This modulated signal is then supplied, as an analog signal, to the matrix switch 5 by the D/A converter 19.
Meanwhile, upon receipt of the picture signal, the analog picture signal supplied to the modem circuit 13 via the telephone interface circuit 6 and the matrix switch 5 is supplied to the multiplier circuit 21. At the same time, the modulated carrier signal is taken out from the analog picture signal in the carrier reproduction circuit 20. Then the modulated carrier signal is supplied to the multiplier 21 where the carrier signal is multiplied by the analog picture signal to perform the demodulating action eliminating the carrier component.
The picture signal thus devoid of the carrier signal is restricted in band by the low-pass filter 22 and is then supplied, as a digital signal, from the A/D converter 23 to the CPU 11.
According to this conventional system, the picture data are modulated into a picture signal in the following manner: As shown in FIG. 7, there are two phases: a first phase of sine curve type, and a second phase of - sine type different by 180.degree. from the first phase, each of the first and second phases being composed of signals having a plurality of amplitudes.
Further, the particular phase and the particular amplitude of one cycle of signal correspond to respective particular brightnesses. In the illustrated example, each phase is eight cycles, i.e., sixteen cycles in total. As is apparent from FIG. 7, the signal "15" of the maximum amplitude of the first phase corresponds to white, and the signal "0" of the maximum amplitude of the second phase corresponds to black. The intermediate signals "14" to "1" correspond to gray of respective brightnesses.
The arrangement of signal points on amplitude and phase planes at that time is shown in FIG. 8.
As described above, a still picture can be transmitted and received during ordinary talking in voice, and hence a mutual understanding at high level can be achieved.
However, in the conventional transmission system for still-picture TV telephone, as is apparent from the sequence diagram of FIG. 6, communication in voice and communication of picture data cannot coexist timewise. When transmitting the picture signal, communication in voice must be temporarily stopped; therefore, smooth and comfortable talking is difficult to achieve.
For example, in the case the picture has a screen size of 160 H.times.100 V picture elements, communication in voice must be stopped for about ten seconds to transmit one picture at a baud rate of 1747.82 Hz. Further, in the system of a high-resolution of 320 H.times.200 V picture elements, communication in voice must be stopped for four times longer than the above time. In the case of a color picture of the above resolution, communication in voice must be stopped for about 1.5 times longer than the above time.